Project Summary Despite the safe sleep campaign, SIDS remains the leading cause of postneonatal infant mortality in the United States today, with an overall rate of 0.40/1000 live births. Our laboratory has provided compelling evidence in 4 independent datasets of defects in the serotonergic (5-HT) network in the medulla (lower brainstem) involved in protective responses to life-threatening challenges during sleep. These 5-HT defects include significantly decreased levels (~26%) of 5-HT itself in SIDS cases compared to age-adjusted controls. Over the last two decades, our group has provided substantial evidence, from studies using neurochemical techniques in frozen tissue, that a subset of SIDS is characterized by serotonergic (5-HT) brainstem pathology in regions involved in cardiorespiratory control and arousal. In 2015, we then reported a novel anatomic finding, from light microscope studies, in the dentate gyrus (DG) of the hippocampus in a major subgroup of SIDS cases (~40%) in a separate dataset. In this latter dataset, 5-HT brainstem neurochemical defects could not be simultaneously sought because the hippocampal tissues were from formalin fixed brains in the archives of the San Diego medical examiner's system. The dentate abnormality in SIDS is part of the spectrum of granule cell dispersion (GCD), which is characterized prominently by bilamination of the granule cells (GCs), so-called dentate bilamination (DB). DB and associated GC abnormalities in the SIDS cases had almost exclusively been reported previously in patients with temporal lobe epilepsy (TLE), some of who died from sudden unexplained death in epilepsy (SUDEP). We now ask: 1) Do cases of SIDS-DB also have 5-HT brainstem pathology? 2) Does the combined entity of SIDS-DB and 5-HT pathology share molecular, cellular, and genetic features with TLE and/or SUDEP? 3) Does SIDS-DB and 5-HT pathology represent one of many entities on a spectrum of underlying, intrinsic temporal lobe vulnerabilities associated with sudden death across life? In these scenarios, DB is postulated to be a developmental abnormality in GC proliferation and/or migration, and a precursor lesion to Ammon's horn sclerosis in TLE. We will test the overall hypothesis that the DG and medullary 5-HT network are both abnormal in the same SIDS cases, and that SIDS cases with DB share molecular, cellular, and genetic features in common with TLE and/or SUDEP. In four Specific Aims, we will test this hypothesis using a variety of state-of-the-art techniques, including transcriptomics with laser capture microdissection in the granule cells of the hippocampus, and whole exome sequencing in SIDS cases with DB. The study's potential impact is the discovery of a cause(s) of large subgroup (40%) of SIDS that will lead to intervention strategies, as well as biomarkers to identify living infants at risk. Understanding DB in SIDS also opens up avenues to elucidate the cellular and molecular pathology of sudden unexplained death beyond infancy, including SUDC and SUDEP.